You’re thinking of the Old Specular system Rasika, whereby you determine with a coloured specular map what colour the specular highlight should be at any given point on the surface. UE4 doesn’t do this (because it’s not how real materials behave), it instead factors in the colour of the light, the colour of the reflection environment and the colour of the diffuse map of the object.
So in effect, there’s nothing to control because it doesn’t exist. It’s done entirely differently.